1. Field of the Invention
The present invention relates to an optical waveguide device and a manufacturing method therefor.
2. Description of the Related Art
An optical device using an optical waveguide has increased in necessity with the evolution of optical communication, and it is used as an optical modulator, optical demultiplexers, optical switch, or optical wavelength converter, for example. Known examples of the optical waveguide include an optical waveguide formed by diffusing titanium (Ti) in a LiNbO3 crystal substrate, an optical waveguide formed by depositing SiO2 on an silicon (Si) Si substrate, and a polymer optical waveguide. As a practical external modulator, a Mach-Zehnder type optical modulator (LN modulator) using a dielectric crystal substrate such as a lithium niobate (LiNbO3) crystal substrate has been developed. Carrier light having a constant intensity from a light source is supplied to the LN modulator to obtain an optical signal intensity-modulated by a switching operation using the interference of light.
The LN modulator includes a dielectric substrate formed from a Z-cut lithium niobate crystal, a pair of optical waveguides formed in the upper surface of the substrate by thermally diffusing titanium (Ti) in the substrate to thereby increase a refractive index, these optical waveguides being combined together near their opposite ends, an SiO2 buffer layer formed on each optical waveguide, and a signal electrode (traveling wave electrode) and a grounding electrode formed on the buffer layers so as to respectively correspond to these optical waveguides. Signal light input from one end of the combined optical waveguides is split at one junction thereof to propagate in the optical waveguides. When a drive voltage is applied to the signal electrode formed over one of the optical waveguides, a phase difference is produced between the split signal lights propagating in the optical waveguides by an electro-optic effect.
In the LN modulator, these signal lights are recombined to be taken out as optical signal outputs. By applying the drive voltage so that the phase difference between the signal lights propagating in the two optical waveguides becomes 0 or π, an on/off pulse signal can be obtained. As a recent LN modulator, the development of a modulator having a high-frequency band of 40 Gb/s has been pursued to realize a higher modulation rate. To reduce a propagation loss and ensure a high-frequency band characteristic in the above-mentioned high-frequency band, it is indispensable to form a groove having a depth of several micrometers between the electrodes along the optical waveguides in the LN modulator. This groove is formed usually by using an RIE (reactive ion etching) dry etching device.
The conventional technique of forming the groove by using the RIE dry etching device has the following problems.
(1) The LN substrate as the base of the LN modulator is a ferroelectric member, so that polarization due to temperature fluctuations occurs and when an electric field on one surface of the substrate reaches about 6,000 V, discharging occurs to cause the damage to a wafer due to discharge shock. In particular, when a sudden temperature change (5° C./min or more) occurs, the damage to the wafer becomes remarkable. The RIE device for use in forming the groove as mentioned above employs a high-frequency power supply, which causes a sudden temperature change to the wafer. As a result, the damage to the wafer easily occurs to cause a reduction in yield.
(2) The RIE device is divided into a load lock chamber (loading chamber) for setting the wafer or taking it out and an etching chamber for actually performing RIE. The wafer is automatically transferred between the load lock chamber and the etching chamber. However, since the LN wafer exhibits a pyroelectric effect, it tends to stick to a metallic member. In a conventional manufacturing method for an LN modulator, the LN wafer sticks to a stage (aluminum electrode) provided in the etching chamber after etching of the LN wafer, so that the wafer cannot be automatically transferred to the load lock chamber.
Accordingly, every time the etching step is ended, the etching chamber is disassembled and a sharp member such as a razor blade is inserted between the wafer and the stage to forcibly separate the wafer from the stage. However, this wafer separating work may easily cause the damage to the wafer, thus remarkably reducing the yield. Further, the etching chamber disassembling work includes a dangerous operation of opening a breaker for the high-frequency power supply and manually disconnecting a signal line. Further, a cooling water pipe, gas induction pipe, etc. must be disconnected, causing a danger and trouble. Further, in reassembling the etching chamber, it is necessary to ensure the assembly accuracy of a wafer chucker, and even if there is a fine positioning error, gas leak occurs to result in generation of a temperature distribution in the LN wafer, thus leading to the damage to the wafer.
(3) In the RIE dry etching, a photoresist is used as a mask, and the LN wafer with the photoresist is etched. At a high temperature (120° C. or higher), the photoresist is erosively burned and oxidized. To prevent this problem, the wafer is cooled through the stage and maintained at a low temperature. Accordingly, a temperature difference is produced between the upper and lower surfaces of the wafer, thus leading to the damage to the wafer.
(4) In the step of patterning exposure of the photoresist for the RIE dry etching, the wafer and a glass mask are aligned. This alignment must be performed with accuracy of 2 μm or less, so as to ensure necessary characteristics. However, since the LN wafer itself is transparent, the luminance at the time of exposure lacks and an alignment marker cannot therefore be viewed, causing a pattern deviation. If the pattern deviation arises, the photoresist must be applied and patterned again, causing a reduction in nonadjusted ratio and a reduction in yield.